This invention relates to body implantable medical devices, and more particularly to defibrillating catheters employing bipolar sensing.
Heart disease is a major cause of deaths in the United States and in other industrial nations. Tachyarrythmias (rapid disturbances in cardiac electrical activity), in particular the conditions of ventricular tachycardia, ventricular flutter and ventricular fibrillation, are widely believed to be the primary cause of sudden deaths associated with heart disease. Atrial tachyarrythmic conditions, on the other hand, are not considered life threatening unless they lead to rapid ventricular disturbance.
Recent experience confirms that tachyarrythmic conditions frequently can be corrected by applying relatively high energy electrical shocks to the heart, a technique often referred to as cardioversion. Cardioversion devices include implantable electronic stand-by defibrillators which, in response to the detection of an abnormally rapid cardiac rhythm, discharge sufficient energy through electrodes connected to the heart to de-polarize and restore the heart to normal cardiac rhythm.
Cardioverting or defibrillation devices typically include means for monitoring heart activity as well as delivery of cardioversion energy. For example, U.S. Pat. No. 3,942,536 (Mirowski et al) discloses an intravascular catheter with a cap electrode at the distal tip, a distal electrode including a plurality of rings near the tip, and a proximal electrode also consisting of a plurality of rings. The tip and distal electrodes are used to provide pacing pulses, while defibrillation pulses are provided using the distal and proximal electrodes. A probe is provided to sense pressure in the right ventricle, to initiate cardioversion upon sensing a pressure that does not exceed a predetermined threshold.
U.S. Pat. No. 4,355,646 (Kallok et al) is directed to a transvenous defibrillating lead with one tip electrode and three additional, annular electrodes. The tip electrode and the most distal of the annular electrodes are placed in the right ventricle and used to measure impedance changes in the ventricle. Defibrillating pulses are delivered across all four of the electrodes.
A key factor in successful defibrillation by implantable devices is the timely and accurate detection of the R-waves, the relatively weak electrical signals produced by ventricular contraction. In particular, the sensing means (one or more electrodes) of the defibrillating device must be capable of quickly detecting abnormally high cardiac rhythm in order to trigger the defibrillation pulse. Perhaps more importantly, the sensing means preferably is able to confirm a successful defibrillation, i.e. a return to normal cardiac rhythm, as soon as possible after each defibrillation pulse. Otherwise, there is the danger of the device delivering an unnecessary and possibly harmful defibrillation pulse.
The advantage of preventing unnecessary or undue defibrillation pulses is recognized in U.S. Pat. No. 4,614,192 (Imran et al). Imran teaches an implantable cardiac defibrillator employing bipolar sensing, in particular a bipolar sensing electrode assembly including a distal tip electrode and a nearby ring electrode, along with two sensing and high voltage delivery electrodes, one in the superior vena cava and another in the form of a patch over the myocardium, near the apex of the heart. This system contemplates three separately implanted electrodes or groups of electrodes. A unitary intravascular multiple electrode catheter is disclosed in U.S. Pat. No. 4,603,705 (Speicher et al). The catheter includes three electrodes: a distal tip electrode, an intermediate spring electrode and a proximal spring electrode. The tip and intermediate electrodes are used in pacing and sensing, while the intermediate and proximal spring electrodes are used to deliver defibrillation pulses.
Use of a common lead for sensing and delivering defibrillation pulses, however, interferes with the timely sensing of R-waves. In particular, tissue proximate the cardioversion discharge electrodes temporarily loses much of its ability to conduct electrical impulses immediately after discharge, resulting in an effective suppression of the R-wave immediately following a defibrillation pulse. Thus, post-shock sensing abnormalities prevent an immediate sensing that the heart has returned to normal sinus rhythm in response to the defibrillation pulse, presenting the risk that another, unneeded defibrillation pulse will be delivered.
Therefore, it is an object of the present invention to provide a unitary intravascular implantable device in which post-defibrillation pulse sensing abnormalities are substantially reduced or eliminated.
Another object is to provide a unitary defibrillation catheter with sensing circuitry independent of the defibrillation circuitry and with increased spacing of sensing electrodes from the nearest defibrillation electrode, for more discrete and localized electrograms.
Another object of the invention is to provide an implantable defibrillation device with a defibrillation pulse delivery system with electrodes and conductors suited for relatively high energy defibrillation, along with independent sensing circuitry including electrodes and conductors suited to sensing.
Yet another object is to provide a unitary defibrillation catheter which simultaneously affords optimum spacing between bipolar sensing electrodes, between a pair of defibrillation electrodes, and between the most adjacent sensing and defibrillation electrodes.